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A rigidity phenomenon for the Hardy-Littlewood maximal function

Stefan Steinerberger (2015)

Studia Mathematica

The Hardy-Littlewood maximal function ℳ and the trigonometric function sin x are two central objects in harmonic analysis. We prove that ℳ characterizes sin x in the following way: Let f C α ( , ) be a periodic function and α > 1/2. If there exists a real number 0 < γ < ∞ such that the averaging operator ( A x f ) ( r ) = 1 / 2 r x - r x + r f ( z ) d z has a critical point at r = γ for every x ∈ ℝ, then f(x) = a + bsin(cx+d) for some a,b,c,d ∈ ℝ. This statement can be used to derive a characterization of trigonometric functions as those nonconstant...

A role of the coefficient of the differential term in qualitative theory of half-linear equations

Pavel Řehák (2010)

Mathematica Bohemica

The aim of this contribution is to study the role of the coefficient r in the qualitative theory of the equation ( r ( t ) Φ ( y Δ ) ) Δ + p ( t ) Φ ( y σ ) = 0 , where Φ ( u ) = | u | α - 1 sgn u with α > 1 . We discuss sign and smoothness conditions posed on r , (non)availability of some transformations, and mainly we show how the behavior of r , along with the behavior of the graininess of the time scale, affect some comparison results and (non)oscillation criteria. At the same time we provide a survey of recent results acquired by sophisticated modifications of the Riccati...

A second order SDE for the Langevin process reflected at a completely inelastic boundary

Jean Bertoin (2008)

Journal of the European Mathematical Society

It was shown in [2] that a Langevin process can be reflected at an energy absorbing boundary. Here, we establish that the law of this reflecting process can be characterized as the unique weak solution to a certain second order stochastic differential equation with constraints, which is in sharp contrast with a deterministic analog.

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